This invention relates to a metallurgical apparatus for converting rod stock or wire rod into wire.
It is known that wire is generally obtained from rod stock or wire rod either manufactured by hot rolling heated billets, or by continuous casting and subsequent rolling, or on the extrusion press. In the following the term "rod stock" will also include wire rod and like material.
Rod stock is generally supplied in diameters from 5 to 10 mm, depending on the type of metal and its end use, and is then converted into wire by means of a drawing process.
The first step of the drawing process is called roughing, and consists of causing the rod stock to pass through a bore in a die (drawplate) having a smaller diameter than the rod stock diameter by applying an external pull or tractive force to the rod stock, and repeating said operation continuously in succession for a sufficient number of times to achieve a final diameter which lies generally in the 1.5-3 mm range. Drawing, therefore, implies stretching the metal, as allowed by the ductile properties of metals and their alloys, said stretching taking place in cold conditions.
Modern drawing machines have reached a high degree of sophistication, but yet unsolved are some of the problems which are typical of the method itself. The critical factor of drawing is in fact the high frictional resistance generated between the die and metal, which notwithstanding the use of highly sophisticated lubrication systems still leads to premature wear of the die and considerable heating of the metal, with attendant limitations to the drawing rate, and therefore the machine output.
Another problem resides in that the wire is required to withstand the entire deformation effort in the tensile form, which brings in substantially two added limitations. Firstly, the tension must be such as not to cause the wire to break, and this limits the maximum contraction that can be achieved with a pass through a given die or drawplate, thereby the number of the successive dies must be increased along with the overall size of the draw system, and secondly, the slightest fault in the wire, such as inclusions, microcracks, etc., in enhanced by the tension state of the wire and leads to the wire breaking, whereby the manual operation of inserting the wire through all of the dies downstream of the break point is to be repeated. That operation requires that the wire end be tapered to a smaller diameter than the die, the wire passed through the die, the draw bench actuated to supply a short section of wire, the wire end re-tapered, and so forth, prior to resuming continuous drawing process.
It should be further noted that during the drawing process, the material undergoes progressive work hardening, directly related, depending on the type of metal, to the sectional reduction undergone. Work hardening creates in the metal an increase of unit breaking load and reduction of the elongation percentage thereof. In excess of a certain amount of work hardening, the elongation percentage is so low as to make it impossible to draw the metal without prior annealing, which may be carried out continuously, but still involves substantial added costs.